FIG. 24 shows a conventional speaker. The conventional speaker has the following elements:                magnetic circuit 1;        voice coil body 4 having coil section 3 movable in magnetic gap 2;        diaphragm 5 of which inner periphery is coupled to voice coil body 4 and outer periphery is coupled to frame 7 via edge 6; and        damper 8 of which inner periphery is coupled to voice coil body 4 and outer periphery is coupled to frame 7.        
In the conventional speaker, when an electric signal output from an audio amplifier or the like is fed into coil section 3, voice coil body 4 vibrates, the vibromotive force is transmitted to diaphragm 5, and diaphragm 5 vibrates air to convert the electric signal to voice. Damper 8 is combined with edge 6 to form a suspension to prevent voice coil body 4 from rolling. Damper 8 has a combined shape of a plurality of corrugations and is hence prevented from working as a movable load on voice coil body 4. Diaphragm 5 is made conical to secure rigidity thereof.
Since damper 8 has the combined shape of the plurality of corrugations in the speaker having this configuration, the movable load of damper 8 has high nonlinearity and high asymmetry between the behavior where diaphragm 5 moves toward magnetic circuit 1 and the behavior where diaphragm 5 moves in the direction opposite to magnetic circuit 1. This speaker therefore has problems related to the following items:                linearity of relation between an input signal of the speaker and displacement of amplitude of diaphragm 5;        symmetry in the vertical direction; and        distortion and quality of sound.        
Since diaphragm 5 is made conical to secure rigidity thereof, it is difficult to thin the speaker itself.
A speaker as one of measures for addressing the problems is disposed in Japanese Patent Unexamined Publication No. 2000-69588. In this speaker, as shown in FIG. 25, two curved dampers 8 are mounted mutually oppositely between voice coil body 4 and frame 7. It is described that using two dampers 8 cancels the nonlinearity of the relation between the input signal level and amplitude of diaphragm 5 and improves the distortion and quality of sound.
In the speaker having the conventional configuration, however, the improvement of the distortion and quality of the sound is not sufficiently effective and thinning of the speaker is difficult.
Using damper 8 causes the problems. The movable load of damper 8 has high nonlinearity and high asymmetry between the behavior where voice coil body 4 moves toward magnetic circuit 1 and the behavior where voice coil body 4 moves in the direction opposite to magnetic circuit 1, so that large harmonic distortion occurs and power linearity degrades. Diaphragm 5 is required to be thick for securing rigidity thereof, so that there is a limit to thin the speaker.
FIG. 26 shows power linearity of the conventional speaker of FIG. 24, namely relation between input power of the speaker and displacement of diaphragm 5. Curve A0 shows an amplitude characteristic of diaphragm 5 directed toward magnetic circuit 1, and curve B0 shows an amplitude characteristic of diaphragm 5 in the direction opposite to magnetic circuit 1. FIG. 27 shows a harmonic distortion characteristic of the conventional speaker, and shows that the larger the dynamic range of the output sound pressure and the harmonic distortion is, the lower the harmonic distortion is. Curve C0 shows an output sound pressure characteristic, curve D0 shows a second harmonic distortion characteristic, and curve E0 shows a third harmonic distortion characteristic.
For addressing the degradation of power linearity and the problem of the harmonic distortion characteristic caused by the nonlinearity and asymmetry, various proposals for addressing the nonlinearity and asymmetry of damper 8 are provided. However, damper 8 has the combined shape of the plurality of corrugations to reduce the movable load, as discussed above. As long as damper 8 is combined with edge 6 to form the suspension, it is difficult to improve performance of the speaker by solving the asymmetry and especially the nonlinearity and reducing the harmonic distortion.
Diaphragm 5 is required to be thick for securing rigidity thereof, the conventional speaker is essentially difficult to be thinned.